Yea i saw this issue when i first got the plugin. What i cant figure out is i notice that it only can minify RSS feeds but it does not cache them. Has anyone ever been able to get RSS feeds to cache? As in an actual feed cache file is generated (what is the name of this file and what directory)? I had to write a patch for this plugin so it could properly page cache rss feeds since out-of-the-box this plugin does not cache rss feeds..it actually renders them in real-time straight from the database. Yikes! Anyway, my fix works well and rss renders are now instant.
Anyway, its been a while and i can’t remember how to resolve the html output problem others may be having hear but off the top of my head you could detect if a rss feed is being generated in your functions.php file and if so then immediately push back the correct “Content Type” header (e.g. application/rss+xml) before this plugin has a chance to push text/html.